A coffee in one shop can contain six times more caffeine than those bought in others, according to researchers from Glasgow University, who analysed levels in 20 premises. They found that a single espresso in one independent cafe contained 322mg, while one from Starbucks had just 51mg, the lowest of the 20.

RCOG spokeswoman Dr Daghni Rajasingam said excess caffeine in pregnancy had been linked to an increased risk of a mother delivering a small baby or having a miscarriage. Small babies are at greater risk of developing heart problems and cardiovascular disease later in life, she added.

Pregnant women should be aware that the caffeine content of coffee varies from shop to shop, should ask how strong a coffee is before they buy it and do their best to keep their intake down to no more than 300mg a day, and only after the first trimester, said Rajasingham.

Dr Euan Paul, executive director of the British Coffee Association, said: "Caffeine content does vary between different blends that are available. The overall advice for coffee drinkers is that 400 - 500mg of caffeine per day is safe and may confer some health benefits but it's important that pregnant women do limit their intake to 200mg per day, from all sources. For pregnant women that are concerned, switching to decaff coffee will ensure that they are drinking less caffeine whilst still enjoying their cup of coffee".